The invention relates to an electrolytic membrane comprising an ion-conducting polymer which is fixed in pores of a polyalkene membrane whose porosity is 30-90% by volume. Such membranes are used, for example, in fuel cells, electrolysis cells, primary and secondary batteries. In particular, such membranes are used at points where a high ion conductivity is desired in combination with a high mechanical strength. Although a high ion conductivity can be obtained by using a membrane having a small thickness, a small thickness is generally achieved at the expense of the strength of the membrane.
Such membranes are disclosed in U.S. Pat. No. 4,849,311. U.S. Pat. No. 4,849,311 describes an electrolytic membrane which contains an ion-conducting polymer which is fixed in the pores of a porous polyethylene membrane. The porosity of the membrane described in U.S. Pat. No. 4,865,930 is between 40 and 90%. The pores have a mean size between 0.001 xcexcm and 0.1 xcexcm. According to the teaching of U.S. Pat. No. 4,849,311, pores having a mean diameter greater than 0.1 xcexcm are difficult to fill and, once filled, the electrolyte easily leaks out again. The membrane is preferably made of polyethylene having a weight-average molar mass of at least 500,000 g/mol. Nafion(copyright) is described as ion-conducting polymer; a perfluorocarbon compound containing a sulphonic acid group.
A disadvantage of such a membrane is that the ion conductivity is relatively low.
The object of the invention is to provide an electrolytic membrane having a higher ion conductivity.
According to the invention, this object is achieved in that the membrane is stretched in at least one direction and has a mean pore size between 0.1 and 5 xcexcM.
The membrane according to the invention is found to have a higher ion conductivity than a known membrane having a smaller pore size and a comparable membrane thickness, porosity and strength.
An advantage of the membrane according to the invention is that it is gastight, as a result of which the membrane is very suitable to be used in a solid-polymer fuel cell.
The membrane according to the invention contains an ion-conducting polymer. Ion-conducting polymers which can be used are known and a few are even commercially available. Suitable polymers are described in Patent Specifications U.S. Pat. Nos. 4,849,311 and 4,865,930. Ion-conducting polymers which are preferably used are polymers based on perfluorosulphonic acid and copolymers of tetrafluoroethylene with perfluorosulphonyl ethoxyvinyl ether are very suitable, the sulphonyl groups being converted into sulphonic acid groups. Such polymers are commercially available under the brand names Nafion and Aciplex. Other examples of suitable ion-conducting polymers are complexes of alkali-metal or alkaline-earth-metal salts with a polar polymer. Examples of these are polyethylene glycol ethers. Complexes of the abovementioned polymers with an ion-donating acid can also be used.
In the membrane according to the invention, the ion-conducting polymer is fixed in the pores of a polyalkene membrane.
Suitable as porous polyalkene membrane are, in particular, porous membranes of polyethylene, polypropylene and ethylene-propylene copolymers. The porosity of the polyalkene membrane according to the invention is between 30 and 90% by volume. This means that the volume of the pores accounts for 30-90% by volume of the total volume of the total membrane.
It has been found that, at a porosity lower than 30%, the ion conductivity of the membrane decreases, while, at a porosity greater than 90%, the mechanical strength decreases undesirably.
The best results are obtained with a polyalkene membrane having a porosity of 60 to 85%.
The membrane according to the invention is stretched in at least one direction and has a mean pore size between 0.1 and 5 xcexcm. In J. Electroanal. Chem. 235 (1987), 299-315, J. Leddy and N. E. Vanderborgh describe how the transport through a Nafion phase, and therefore also the ion conductivity, decreases at pore diameters greater than 0.05 xcexcm. Surprisingly it was found that, indeed, a membrane stretched in at least one direction having a pore diameter greater than 0.05 xcexcm has a better ion conductivity than a membrane having a pore diameter smaller than 0.05 xcexcm.
With a mean pore diameter greater than 5.0 xcexcm, the ion-conducting polymer can no longer be fixed in the pores because the pores are then too large. The best results are obtained with a pore diameter of 0.15 to 2.5 xcexcm.
The electrolytic membrane according to the invention has a good mechanical strength, as a result of which no cracks occur even in the case of relatively thin membranes if the membrane is processed for its application. The electrolytic membrane according to the invention preferably has a tensile strength of at least 15 MPa, while the thickness of the membrane may vary from 15 to 150 xcexcm, preferably from 20 to 60 xcexcm. With such a thickness, the membrane according to the invention has an ion conductivity of at least 0.0004 S/cm, but the ion conductivity is preferably considerably higher, i.e. at least 0.0008 S/cm, while a particularly suitable membrane has an ion conductivity of 0.002 to 0.08 S/cm.
The invention also relates to a method of manufacturing the electrolytic membrane according to the invention.
Such a method is disclosed in U.S. Pat. No. 4,849,311. In it, the pores of a porous membrane, preferably polyalkene membrane, are filled by means of capillary condensation with a solution of an ion-conducting polymer.
A disadvantage of the method described in U.S. Pat. No. 4,849,311 is that it is difficult to manufacture an electrolytic polyalkene membrane whose pores have a mean diameter of more than 0.1 xcexcm. According to the said patent specification, the mean pore diameter should preferably be even less than 0.025 xcexcm. The reason given for this is that the ion-conducting polymer leaks away from pores greater than 0.1 xcexcm so that such a membrane is unstable.
The object of the invention is to eliminate said disadvantage completely or partially.
According to the invention said object is achieved by a method which comprises the following steps:
a) dissolving an ion-conducting polymer in a solvent, at least 25% by weight of which is composed of a component having a boiling point higher than 125xc2x0 C.,
b) applying an amount of the solution prepared under (a) to a horizontal polyalkene membrane stretched in at least one direction and having a pore volume of 30-90% of the total volume of the membrane, the amount of the solution being chosen in such a way that the volume of the ion-conducting polymer present therein is more than 60% of the pore volume, and the membrane being sealed at the underside,
c) evaporating the solvent at a temperature which is at least 80xc2x0 C. and which is lower than the melting point of the polyalkene membrane.
A stable electrolytic membrane having an ion-conducting polymer fixed in the pores of a polyalkene membrane having a mean pore size between 0.1 and 5.0 xcexcm is manufactured by the method according to the invention.
An advantage of the method according to the invention is that the ion-conducting polymer which is present in the solution on the horizontal stretched membrane becomes preferentially concentrated at the polyalkene surface. Since most of the surface is in the pores, the ion-conducting polymer becomes concentrated in the pores of the membrane. A method in which partially filled pores need to be refilled with a fresh solution can thereby be avoided. The disadvantage of refilling pores is that air inclusions are easily produced. The refilling of partially filled pores is therefore only possible in the presence of solvents which have a low wetting angle with the polyalkene. Such a limitation does not apply in the case of the method according to the invention.
A further advantage of the method according to the invention is that a porous membrane having a pore size above 0.1 xcexcm is easy to impregnate, as a result of which virtually no air is enclosed in the membrane.
In the method according to the invention, an ion-conducting polymer is dissolved in a solvent, at least 25% by weight of which is composed of a component having a boiling point higher than 125xc2x0 C. Solutions of ion-conducting polymers can be prepared as described in, for example, U.S. Pat. No. 4,849,311. A good method of obtaining a suitable solution of an ion-conducting polymer is, for example, heating the solid polymer in a suitable solvent. Suitable solvents for the preferred polymers perfluorosulphonic acid-based polymers are water and alcohols such as ethanol, n-propanol or isopropanol. The presence of a component having a boiling point of more than 125xc2x0 C. ensures that the evaporation takes place slowly. This, in combination with a particular pore structure of a stretched membrane, results in the ion-conducting polymer no longer, or scarcely any longer, dissolving after the solvent has evaporated. As a result, a stable membrane is obtained. Suitable components having a boiling point of more than 125xc2x0 C. are, for example, ethylene glycol, N,N-dimethylformamide and dimethyl sulphoxide. Preferably, the component having a boiling point higher than 125xc2x0 C. in the solvent and the amount thereof are chosen in such a way that the evaporation takes between 10 and 24 hours.
Preferably, at least 50% by weight of the solvent is composed of a component having a boiling point higher than 150xc2x0 C. This ensures that the evaporation can take place at a higher temperature without the evaporation time being substantially shortened as a consequence. A further advantage of a component having a boiling point higher than 150xc2x0 C. is that less of it is necessary to arrive at the desired evaporation time than of a solvent having a lower boiling point. Consequently, the choice of solvent is less critical. In particular, if more than 50% by weight of the component having a boiling point higher than 150xc2x0 C. is present, higher requirements are imposed on the solubility of the ion-conducting polymer in said component than if said percentage is significantly below 50% by weight. Preferably, the component having a boiling point higher than 150xc2x0 C. is dimethyl sulphoxide (DMSO). The advantage of DMSO is that it has a boiling point of 189xc2x0 C. and is not toxic.
A suitable concentration of the ion-conducting polymer in the solvent is between 2 and 25% by weight. Preferably, a solution is used in which the concentration of ion-conducting polymer is 2-5% by weight.
It has been found that slow evaporation at a temperature which is as little as possible below the melting point of the polyalkene membrane adds to the insolubility of the solid ion-conducting polymer formed in the pores.
In the method according to the invention, an amount of the abovementioned solution is applied to a horizontal polyalkene membrane stretched in at least one direction and having a pore volume of 30-90% of the total volume of the membrane, the amount of the said solution being chosen so that the volume of the ion-conducting polymer present therein is more than 60% of the pore volume and the membrane being sealed at the underside.
This ensures that, after the evaporation of the solvent, at least nearly 60% by volume of the pores are filled with the ion-conducting polymer. The fact that a small portion of the ion-conducting polymer remains behind on the surface of the membrane has the result that the amount of ion-conducting polymer in the pores is somewhat below the amount calculated on the basis of the pore volume. Preferably, the amount of the solution is chosen in such a way that at least 80% by volume, or still more preferably, at least 95% by volume of the pores are filled with the ion-conducting polymer. Since, as described above, the ion-conducting polymer preferentially deposits on the polyalkene surface and the total pore surface is many times greater than the outer surfaces of the membrane, the ion-conducting polymer which deposits on the outer surface of the membrane can be neglected in the calculation of the amount of solution containing ion-conducting polymer.
A measure for facilitating the impregnation is first to wet the membrane with the solvent prior to impregnating it with the solution of the ion-conducting polymer.
Polyalkene membranes which are stretched in at least one direction are disclosed in EP-A-504,954. EP-A-504,954 describes a method of preparing a polyalkene membrane from a solution of a polyalkene in a volatile solvent. The membrane passes through a cooling bath containing a coolant and the solvent is removed at a temperature below the temperature at which the polyalkene dissolves in the solvent, after which the membrane is stretched in at least one direction. If the polyalkene is polyethylene, the weight-average molecular weight may vary between 100,000 and 5,000,000 g/mol. Preferably, the membrane contains polyethylene having a weight-average molecular weight which is less than 500,000 g/mol. It has been found that, in the presence of polyethylene having such a molecular weight, it is easy to manufacture membranes having an mean pore size between 0.1 and 5.0 xcexcm. Suitable, in particular, are mixtures of polyethylene with various molecular weights. Thus, a mixture which contains polyethylene having a weight-average molecular weight less than 500,000 g/cm3 can also contain polyethylene having a weight-average molecular weight greater than 1,000,000 g/cm3. As a result of the presence of the latter polyethylene, a high strength of the membrane is obtained.
During the application of the solution of the ion-conducting polymer, the membrane is sealed at the underside. This prevents the solution applied to the membrane from leaving the membrane at the underside and thus from being lost for deposition in the pores of the membrane. The sealing of the membrane at the underside can be effected by, for example, clamping the membrane onto a flat plate.
The solution applied to the membrane is then evaporated at a temperature which is at least 80xc2x0 C. and which is lower than the melting point of the polyalkene membrane. Preferably, the temperature at which the solvent is evaporated is more than 110xc2x0 C., the evaporation time being, as already described above, at least 10 hours. It has been found that the higher the temperature at which evaporation is carried out, the lower is the solubility of the solid ion-conducting polymer formed in the H+ form.
Before using the membrane according to the invention, for example in a solid-polymer fuel cell, it should be cleaned. This is possible in various ways and the way in which it is done is not essential for the invention. Any known cleaning procedure can therefore be used. Successive or alternating cleaning with water (Millipore-filtered, 18 megaohmxc2x7cm, ultrasound), hydrogen peroxide solution and sulphuric acid solution is a very suitable method.
The electrolytic membranes according to the invention are very suitable for use in fuel cells and, in particular, in solid-polymer fuel cells or polymer-electrolyte fuel cells. Such fuel cells are known per se. Another use of the electrolytic membrane according to the invention is in batteries. The invention also relates to fuel cells and batteries in which the electrolytic membrane is used.
A solid-polymer fuel cell comprises a porous platinum anode and a porous platinum cathode which are both in contact with the electrolytic membrane. In the standard H2/O2 polymer fuel cell, H2 is oxidized at the anode to H+ ions which are transported through the membrane to the cathode. Water is then formed at the cathode by reduction of O2 in the presence of H+. As a result of the electroosmotic effect, at the anode, water is likewise removed together with H+ through the membrane to the cathode. As a result, there is a risk that drying-out of the anode will occur. In order to combat such drying-out of the membrane, the reactant gases are moistened. A solid-polymer fuel cell is usually operated at a temperature of around 80xc2x0 C.
The invention is further illustrated by reference to the examples below.